Violapyrones A-G, α-pyrone derivatives from Streptomyces violascens isolated from Hylobates hoolock feces

Diaporpyrone E, an undescribed α-pyrone from the endophytic fungus Diaporthe sp. CB10100

An undescribed α-pyrone diaporpyrone E (1), and three known nucleotides, 5'-O-acetyl uridine (2), 5'-O-acetyl thymidine (3), and adenine (4), were identified from Diaporthe sp. CB10100, an endophytic fungus isolated from the medicinal plant Sinomenium acutum. The structure of 1 was determined by extensive analysis of its HRMS, 1D and 2D NMR spectroscopic data, as well as electronic circular dichroism calculations and comparison. The in vitro cytotoxic and antibacterial assays of 1 revealed that it has a 30.2% inhibitory effect on HepG2 cells at 50 μM, while no antibacterial activities against Staphylococcus aureus and Klebsiella pneumoniae at 64 μg/mL.

Functional analysis of a putative type III polyketide synthase from deep-sea sediment metagenome

Type III polyketide synthases (type III PKSs) are single homodimeric enzymes that produce diverse products such as phloroglucinol, pyrones, resorcinols and chalcones which are biotechnologically important molecules. In an attempt to identify new type III PKS from extreme environments, the deep-sea sediment metagenome from Bay of Bengal was screened for type III PKS genes. BLASTX analyses of Nanopore sequence derived metagenome with the in-house created PKS database revealed a full length type III PKS from a 5 kb fragment. The annotated full length type III PKS, S9PKS showed 25-30 % sequence identity towards previously characterized enzymes. To functionally characterize the gene, it was synthesized, cloned into pET28a and pColdI vectors under T7 and csp promoters, respectively, and expressed in Escherichia coli Rosetta(DE3) pLysS. The optimized PKS (OptiPKS) was expressed as inclusion bodies under both promoters. The inclusion bodies were successfully solubilised using low concentration of urea, refolded and purified using Ni-NTA Agarose resin. The purified OptiPKS was tested for functionality using fatty acyl-CoA substrates at various temperatures. High performance liquid chromatography (HPLC) analyses revealed that OptiPKS produced tri and tetraketide pyrones using C4 to C10 acyl-CoA starter substrates. Further characterization and mutation of the enzyme would reveal its functional significance. Thus, the study could be a lead for the annotation and functional characterization of putative type III PKS from environmental metagenome data.

Two-step, high-yielding total synthesis of antibiotic pyrones

A simple two-step dialkylation protocol was developed to synthesize biologically active antibiotics photopyrones, pseudopyronines, and violapyrones from bio-renewable triacetate lactone in excellent yields. These pyrones are functionally modified into another set of pyrone natural products by a single O-methylation reaction. The high-yielding gram scale synthesis of four natural products [pseudopyronine A, photopyrone A, pseudopyronine B and photopyrone C] demonstrated the viability for industrial applications.

Two pairs of 2-pyrone enantiomers and a benzophenone analogue from the endophytic fungus Penicillium egyptiacum

Four new 2-pyrone derivatives, two pairs of enantiomers, (±)-egypyrone A [(±)-1] and (±)-egypyrone B [(±)-2], together with a new benzophenone analogue, orbiophenone B (3), were isolated from the endophytic fungus Penicillium egyptiacum. The enantiomeric mixtures (±)-1 and (±)-2 were separated through chiral HPLC, respectively. Their structures were elucidated by extensive analysis of spectroscopic data and the absolute configuration was determined by comparing the optical rotation of structurally similar molecule. Subsequently, the cytotoxic activities of (±)-1, (±)-2, and 3 against the U87 cell line were tested and no activity was observed at a concentration of 10 µM.

Ethylidene-Tethered Chromene-Pyrone Hybrids as Potential Plant-Growth Regulators from an Endolichenic Phaeosphaeria Species

Phaeosphaeria sp., a lichen-associated fungus, produced six skeletally new dimeric spiciferones (1-6) and four known metabolites (7-10). The new structures were elucidated by spectroscopic analysis, and their absolute configurations were determined by electronic circular dichroism calculations. Compounds 1 and 3-6 represent the first examples of ethylidene-bridged dimers from the building blocks 4H-chromene-4,7(8H)-dione and α-pyrone, and 2 is a unique homodimer of spiciferone. Compounds 1, 2, and 5-9 significantly inhibited the growth of weed-like dicot Arabidopsis thaliana at 100.0 μM. Notably, 8 showed the strongest inhibitory activity against the fresh weight and root elongation of A. thaliana with the IC₅₀ values of 32.04 and 26.78 μM, respectively, whereas 1, 8, and 9 stimulated the growth of A. thaliana at lower concentrations. Meanwhile, compounds 2 and 6 exhibited weak inhibitory effects on the root elongation of monocot rice, while 1 and 8 exhibited growth-promoting effects on the shoot and root elongation of rice in a roughly dose-dependent manner.

Gut and Gill-Associated Microbiota of the Flatfish European Plaice (Pleuronectes platessa): Diversity, Metabolome and Bioactivity against Human and Aquaculture Pathogens

Similar to other marine holobionts, fish are colonized by complex microbial communities that promote their health and growth. Fish-associated microbiota is emerging as a promising source of bioactive metabolites. Pleuronectes platessa (European plaice, plaice), a flatfish with commercial importance, is common in the Baltic Sea. Here we used a culture-dependent survey followed by molecular identification to identify microbiota associated with the gills and the gastrointestinal tract (GIT) of P. platessa, then profiled their antimicrobial activity and metabolome. Altogether, 66 strains (59 bacteria and 7 fungi) were isolated, with Proteobacteria being the most abundant phylum. Gill-associated microbiota accounted for higher number of isolates and was dominated by the Proteobacteria (family Moraxellaceae) and Actinobacteria (family Nocardiaceae), whereas Gram-negative bacterial families Vibrionaceae and Shewanellaceae represented the largest group associated with the GIT. The EtOAc extracts of the solid and liquid media cultures of 21 bacteria and 2 fungi representing the diversity of cultivable plaice-associated microbiota was profiled for their antimicrobial activity against three fish pathogens, human bacterial pathogen panel (ESKAPE) and two human fungal pathogens. More than half of all tested microorganisms, particularly those originating from the GIT epithelium, exhibited antagonistic effect against fish pathogens (Lactococcus garvieae, Vibrio ichthyoenteri) and/or human pathogens (Enterococcus faecium, methicillin-resistant Staphylococcus aureus). Proteobacteria represented the most active isolates. Notably, the solid media extracts displayed higher activity against fish pathogens, while liquid culture extracts were more active against human pathogens. Untargeted metabolomics approach using feature-based molecular networking showed the high chemical diversity of the liquid extracts that contained undescribed clusters. This study highlights plaice-associated microbiota as a potential source of antimicrobials for the control of human and the aquaculture-associated infections. This is the first study reporting diversity, bioactivity and chemical profile of culture-dependent microbiota of plaice.

Anti-cancer and antimicrobial potential of five soil Streptomycetes: a metabolomics-based study

Lack of new anti-cancer and anti-infective agents directed the pharmaceutical research to natural products' discovery especially from actinomycetes as one of the major sources of bioactive compounds. Metabolomics- and dereplication-guided approach has been used successfully in chemical profiling of bioactive actinomycetes. We aimed to study the metabolomic profile of five bioactive actinomycetes to investigate the interesting metabolites responsible for their antimicrobial and anti-cancer activities. Three actinomycetes, namely, Streptomyces sp. SH8, SH10 and SH13, were found to exhibit broad spectrum of antimicrobial activities, whereas isolate SH4 showed the broadest antimicrobial activity against all tested strains. In addition, isolates SH8, SH10 and SH12 displayed potent cytotoxicity against the breast cancer cell line Michigan Cancer Foundation-7 (MCF-7), whereas isolates SH4 and SH12 exhibited potent anti-cancer activity against the hepatoma cell line hepatoma G2 (HepG2) compared with their weak inhibitory properties on the normal breast cells MCF-10A and normal liver cells transformed human liver epithelial-2 (THLE2), respectively. All bioactive isolates were molecularly identified as Streptomyces sp. via 16S rRNA gene sequencing. Our actinobacterial dereplication analysis revealed putative identification of several bioactive metabolites including tetracycline, oxytetracycline and a macrolide antibiotic, novamethymycin. Together, chemical profiling of bioactive Streptomycetes via dereplication and metabolomics helped in assigning their unique metabolites and predicting the bioactive compounds instigating their diverse bioactivities.

Comoclathrin, a novel potent skin-whitening agent produced by endophytic Comoclathris strains associated with Andalusia desert plants

As part of our screening program for the discovery of molecules of microbial origin with skin-whitening activity, 142 diverse fungal endophytes from a wide variety of Andalusia arid plants were screened, applying the OSMAC approach. The fungal strains CF-090361 and CF-090766, isolated from xerophytic plants, were selected as the most promising, while phylogenetic analysis revealed that both strains could represent a new species within the genus Comoclathris. The effect of different fermentation conditions on the production of tyrosinase inhibitory activity was examined, in order to identify the optimum cultivation conditions. LCMS based metabolomics was applied to determine significant differences between the strains and fermentation conditions, and to identify potential bioactive secondary metabolites. Bioassay-guided purification of the main active components led to the isolation of three new compounds (1-3), along with the known compounds graphostrin B (4) and brevianamide M (5). Compound 1 (Comoclathrin) demonstrated the strongest anti-tyrosinase activity (IC50 0.16 μΜ), which was 90-times higher than kojic acid (IC50 14.07 μΜ) used as positive control. Additionally, comoclathrin showed no significant cytotoxicity against a panel of cancer cell lines (HepG2, A2058, A549, MCF-7 and MIA PaCa-2) and normal BJ fibroblasts. These properties render comoclathrin an excellent development candidate as whitening agent.

Synthesis of C2-Symmetrical Bis-(β-Enamino-Pyran-2,4-dione) Derivative Linked via 1,6-Hexylene Spacer: X-ray Crystal Structures, Hishfeld Studies and DFT Calculations of Mono- and Bis-(Pyran-2,4-diones) Derivatives

The synthesis of C2-symmetrical bis(β-enamino-pyran-2,4-dione) derivative 3 connected via 1,6-hexylene linker was reported for the first time. X-ray structures and Hirshfeld studies of the new bis- β-enamino-pyran-2,4-dione derivative 3 along with two structurally related pyran-2,4-dione derivatives 2a,b were discussed. A comparative analysis of the different intermolecular contacts affecting the crystal stability was presented. Generally, the H…H, O…H, and H…C interactions are common in all compounds and are considered the most abundant contacts. In addition, DFT calculations were used to compute the electronic properties as well as the 1H and 13C NMR spectra of the studied systems. All compounds (except 3) are polar where 2a (3.540 Debye) has a higher dipole moment than 2b (2.110 Debye). The NMR chemical shifts were calculated and excellent correlations between the calculated and experimental data were obtained (R2 = 0.93–0.94).

Bioactive α-Pyrone Derivatives from the Endophytic Fungus Diaporthe sp. CB10100 as Inducible Nitric Oxide Synthase Inhibitors

Inducible nitric oxide synthase (iNOS) produces NO from l-arginine and plays critical roles in inflammation and immune activation. Selective and potent iNOS inhibitors may be potentially used in many indications, such as rheumatoid arthritis, pain, and neurodegeration. In the current study, five new compounds, including a dibenzo-α- pyrone derivative ellagic acid B (5) and four α-pyrones diaporpyrone A-D (9-12), together with three known compounds (6-8), were isolated from the endophytic fungus Diaporthe sp. CB10100. The structures of these new natural products were unambiguously elucidated using NMR, HRESIMS or electronic circular dichroism calculations. Ellagic acid B (5) features a tetracyclic 6/6/6/6 ring system with a fused 2H-chromene, which is different from ellagic acid (4) with a fused 2H-chromen-2-one. Both 2-hydroxy-alternariol (6) and alternariol (7) reduced the expression of iNOS at protein levels in a dose-dependent manner, using a lipopolysaccharide (LPS)-induced RAW264.7 cell models. Also, they decreased the protein expression levels of pro-inflammatory cytokines, such as tumor necrosis factor-α, interleukin-6 and monocyte chemotactic protein 1. Importantly, 6 and 7 significantly reduced the production of NO as low as 10 μM in LPS-induced RAW264.7 cells. Molecular docking of 6 and 7 to iNOS further suggests that both of them may interact with iNOS. Our study suggests that 6 and 7, as well as the alternariol scaffold may be further developed as potential iNOS inhibitors.

Penipyrols C-G and methyl-penipyrol A, α-pyrone polyketides from the mangrove derived fungus Penicillium sp. HDN-11-131

Six new α-pyrone polyketides, penipyrols C-G (1-5) and methyl-penipyrol A (6), together with one biogenetically related known compound, penipyrol A (7), were isolated from the extract of fungus Penicillium sp. HDN-11-131. Their structures including the absolute configurations were established by extensive spectroscopic analysis, Mosher's method, and ECD calculations as well as biogenic considerations. Compounds 1-4 possess a rare skeleton featuring γ-butyrolactone linked to α-pyrone ring through double bond. Compound 1 can induce pancreatic β-cell regeneration in zebrafish at 10 μM, which demonstrated promising anti-diabetes potential.

2-Pyrones from endophytic fungus Diaporthe foeniculina BZM-15

Foeniculins A-C (1-3) together with a pair of enantiomers (±)-foeniculin D (4) were isolated from endophytic fungus Diaporthe foeniculina BZM-15. Their structures including absolute configurations were unambiguously established by extensive interpretation of the NMR and HR-ESI-MS data, ECD measurements powered by molecular calculations, as well as Mo₂(OAc)₄ mediated CD methodology. The cytotoxic activity assay disclosed that these compounds didn't show any noticeable cytotoxic activity.

Antiviral properties of extracts of Streptomyces sp. SMU 03 isolated from the feces of Elephas maximus

Actinobacteria are historically and continued to be an important source for drug discovery. The annual epidemics and periodic pandemics of humans induced by influenza A virus (IAV) prompted us to develop new effective antiviral drugs with different modes of action. An actinobacterium of Streptomyces sp. SMU 03 was identified from the feces of Elephas maximus in Yunnan Province, China. By employing an H5N1 pseudo-typed virus drug screening system, the anti-IAV effect of the dichloromethane extracts (DCME) of this bacterium was investigated. DCME showed broad and potent activities against several influenza viruses, including the H1N1 and H3N2 subtypes and influenza B virus, with IC₅₀ values ranging from 0.37 ± 0.22 to 14.44 ± 0.79 μg/mL. A detailed modes-of-action study indicated that DCME might interact with the HA2 subunit of hemagglutinin (HA) of IAV by interrupting the fusion process between the viral and host cells' membranes thereby inhibiting the entry of the virus into host cells. Furthermore, the in vivo anti-IAV activity test of DCME showed that compared with the no-drug treated group, the survival rates, appearances, weights, lung indices and histopathological changes were all significantly alleviated. Based on these results, the chemical constituent study of DCME was then investigated, from which a number of antiviral compounds with various structural skeletons have been isolated and identified. Overall, these data indicated that the DCME from Streptomyces sp. SMU 03 might represent a good source for antiviral compounds that can be developed as potential antivirus remedies.

Three new α-pyrone derivatives from the plant endophytic fungus Penicillium ochrochloronthe and their antibacterial, antifungal, and cytotoxic activities

Three new 3,4,6-trisubstituted α-pyrone derivatives, namely 6-(2'R-hydroxy-3'E,5'E-diene-1'-heptyl)-4-hydroxy-3-methyl-2H-pyran-2-one (1), 6-(2'S-hydroxy-5'E-ene-1'-heptyl)-4-hydroxy-3-methyl-2H-pyran-2-one (2), and 6-(2'S-hydroxy-1'-heptyl)-4 -hydroxy-3-methyl-2H-pyran-2-one (3), together with one known compound trichodermic acid (4), were isolated from the solid-substrate fermentation culture of Penicillium ochrochloronthe associated the roots of Taxus media. Compounds 1-4 displayed the antimicrobial activity selectively against tested fungal and bacterial strains with minimum inhibitory concentration (MIC) values ranging from 12.5 to 100 μg/ml. Furthermore, we found that only compound 4 exhibited moderate cytotoxicity against five human cancer cells (A549, LN229, MGC, LOVO, and MDA231) with IC₅₀ values of 51.45, 23.43, 39.16, 46.97, and 42.85 μg/ml, respectively.

The value of universally available raw NMR data for transparency, reproducibility, and integrity in natural product research

Covering: up to 2018With contributions from the global natural product (NP) research community, and continuing the Raw Data Initiative, this review collects a comprehensive demonstration of the immense scientific value of disseminating raw nuclear magnetic resonance (NMR) data, independently of, and in parallel with, classical publishing outlets. A comprehensive compilation of historic to present-day cases as well as contemporary and future applications show that addressing the urgent need for a repository of publicly accessible raw NMR data has the potential to transform natural products (NPs) and associated fields of chemical and biomedical research. The call for advancing open sharing mechanisms for raw data is intended to enhance the transparency of experimental protocols, augment the reproducibility of reported outcomes, including biological studies, become a regular component of responsible research, and thereby enrich the integrity of NP research and related fields.

Emerones A–C: three novel merosesquiterpenoids with unprecedented skeletons from Emericella sp. XL029

Three novel merosesquiterpenoids (1–3) from Emericella sp. XL029.

Generation of methylated violapyrones with improved anti-influenza A virus activity by heterologous expression of a type III PKS gene in a marine Streptomyces strain

Heterologous expression of the type III polyketide synthase (PKS) gene vioA in marine-derived Streptomyces youssoufiensis OUC6819 led to production of six violapyrones (VLPs), including four novel compounds VLPs Q-T (1-4) and two known compounds VLPs B and I (5 and 6). The structures of 1-4 were elucidated by a combination of spectroscopic analyses, including HR-ESIMS and 1D and 2D NMR data, demonstrating that 1-4 are novel VLPs which are methylated at 4-OH with their corresponding non-methylated counterparts to be VLP A, 5 and 6 and VLP C, respectively. Anti-influenza A [H1N1 (A/Virginia/ATCC1/2009) and H3N2 (A/Aichi/2/1968)] virus activity of compounds 1-6 as well as VLPs A and C were then evaluated using ribavirin as a positive control (IC₅₀ = 66.7 and 99.6 μM). The results revealed that these VLPs showed considerable anti-H1N1 and anti-H3N2 activities with IC₅₀ values of 30.6-132.4 μM and 45.3-150.0 μM, respectively. Notably, all the methylated VLPs displayed better anti-virus activity than their non-methylated counterparts, among which compound 3 (VLP S) exhibited the best activities. Interestingly, methylation at 4-OH has negative effect on the anti-MRSA (methicillin-resistant Staphylococcus aureus) activity instead, with methylated VLPs displaying decreased (2) or abolished (3 and 4) activities in comparison with each of their non-methylated counterparts.

Overexpression of a type III PKS gene affording novel violapyrones with enhanced anti-influenza A virus activity

BACKGROUND

Type III polyketide synthases (PKSs) are simple homodimer ketosynthases that distribute across plants, fungi, and bacteria, catalyzing formation of pyrone- and resorcinol-types aromatic polyketides with various bioactivities. The broad substrate promiscuity displayed by type III PKSs makes them wonderful candidates for expanding chemical diversity of polyketides.

RESULTS

Violapyrone B (VLP B, 10), an α-pyrone compound produced by deepsea-derived Streptomyces somaliensis SCSIO ZH66, is encoded by a type III PKS VioA. We overexpressed VioA in three different hosts, including Streptomyces coelicolor M1146, Streptomyces sanyensis FMA as well as the native producer S. somaliensis SCSIO ZH66, leading to accumulation of different violapyrone compounds. Among them, S. coelicolor M1146 served as the host producing the most abundant violapyrones, from which five new (2-4, 7 and 12) and nine known (1, 5, 6, 8-11, 13 and 14) compounds were identified. Anti-influenza A (H1N1) virus activity of these compounds was then evaluated using ribavirin as a positive control (IC₅₀ = 112.9 μM), revealing that compounds 11-14 showed considerable activity with IC₅₀ values of 112.7, 26.9, 106.7 and 28.8 μM, respectively, which are significantly improved as compared to that of VLP B (10) (IC₅₀ > 200 μM). The productions of 10 and 13 were increased by adding P450 inhibitor metyrapone. In addition, site-directed mutagenesis experiment led to demonstration of the residue S242 to be essential for the activity of VioA.

CONCLUSIONS

Biological background of the expression hosts is an important factor impacting on the encoding products of type III PKSs. By using S. coelicolor M1146 as cell factory, we were able to generate fourteen VLPs compounds. Anti-H1N1 activity assay suggested that the lipophilic nature of the alkyl chains of VLPs plays an important role for the activity, providing valuable guidance for further structural optimization of VLPs.

Application of Fe3O4@SiO2/(CH2)3-[imidazolium-SO3H]Cl as a robust, magnetically recoverable solid acid catalyst for the facile preparation of arylbispyranylmethanes

In this study, Fe3O4@SiO2/(CH2)3-[imidazolium-SO3H]Cl shows robust promoting capability in the synthesis of arylbispyranylmethane derivatives under mild and green conditions. Arylbispyranylmethanes were synthesized via efficient three-component reaction of various aromatic aldehydes with 4-hydroxy-6-methyl-2H-pyran-2-one. The nanomagnetic core-shell catalyst presented effective potential of at least eight times recycling applicability in the described synthetic procedure.

Saccharomonopyrones A-C, New α-Pyrones from a Marine Sediment-Derived Bacterium Saccharomonospora sp. CNQ-490

Intensive study of the organic extract of the marine-derived bacterium Saccharomonospora sp. CNQ-490 has yielded three new α-pyrones, saccharomonopyrones A-C (1-3). The chemical structures of these compounds were assigned from the interpretation of 1D, 2D NMR and mass spectrometry data. Saccharomonopyrone A (1) is the first α-pyrone microbial natural product bearing the ethyl-butyl ether chain in the molecule, while saccharomonopyrones B and C possess unusual 3-methyl and a 6-alkyl side-chain within a 3,4,5,6-tetrasubstituted α-pyrone moiety. Saccharomonopyrone A exhibited weak antioxidant activity using a cation radical scavenging activity assay with an IC₅₀ value of 140 μM.

A Concise Synthesis of Rhodanthpyrone A and B, Natural 4-(Hydroxyphenyl)-substituted α-Pyrones

A concise synthesis of rhodanthpyrone A and B was accomplished via a Suzuki coupling reaction. To find the conditions appropriate to install hydroxyphenyl moieties to the α-pyrone skeleton, a model study was conducted using commercially available boronic acids. It was revealed that the hydroxy moiety of the phenylboronic acids should be concealed when reacted with labile 4-tosyl α-pyrone. Consequently, rhodanthpyrone A and B could be synthesized in high yields by Suzuki reaction using TIPS-protected arylboronic acids. This procedure provided a concise and versatile route for the synthesis of rhodanthpyrones and their 4-aryl substituted α-pyrone analogs.

Pyran Rings Containing Polyketides from Penicillium raistrickii

Five new pyran rings containing polyketides, penicipyrans A–E (1–5), together with the known pestapyrone A (6), were isolated from the saline soil-derived Penicillium raistrickii. Their structures were determined by interpretation of NMR and HRESIMS data. The absolute configurations of compounds 4 and 5 were established by the modified Mosher’s method and single-crystal X-ray diffraction analysis, respectively. These compounds possessed high structural diversity including two α-pyrones (1, 2), three isocoumarins (3, 4, 6), and one dihydropyran derivative (5). Among them, Compound 5 exhibited cytotoxicity against HL-60 and K562 cell lines with IC₅₀ values of 4.4 and 8.5 μM, respectively.

Structure determination of two new nerolidol-type sesquiterpenoids from the soil actinomycete Streptomyces scopuliridis

Two new nerolidol-type sesquiterpenes were isolated from the culture broth of Streptomyces scopuliridis, and identified based on 1D and 2D NMR spectroscopic data analyses.

Activation of a plasmid-situated type III PKS gene cluster by deletion of a wbl gene in deepsea-derived Streptomyces somaliensis SCSIO ZH66

Background

Actinomycete genome sequencing has disclosed a large number of cryptic secondary metabolite biosynthetic gene clusters. However, their unavailable or limited expression severely hampered the discovery of bioactive compounds. The whiB-like (wbl) regulatory genes play important roles in morphological differentiation as well as secondary metabolism; and hence the wblA_so gene was probed and set as the target to activate cryptic gene clusters in deepsea-derived Streptomyces somaliensis SCSIO ZH66.

Results

wblA_so from deepsea-derived S. somaliensis SCSIO ZH66 was inactivated, leading to significant changes of secondary metabolites production in the ΔwblA_so mutant, from which α-pyrone compound violapyrone B (VLP B) was isolated. Subsequently, the VLP biosynthetic gene cluster was identified and characterized, which consists of a type III polyketide synthase (PKS) gene vioA and a regulatory gene vioB; delightedly, inactivation of vioB led to isolation of another four VLPs analogues, among which one was new and two exhibited improved anti-MRSA (methicillin-resistant Staphylococcus aureus, MRSA) activity than VLP B. Moreover, transcriptional analysis revealed that the expression levels of whi genes (whiD, whiG, whiH and whiI) and wbl genes (wblC, wblE, wblH, wblI and wblK) were repressed by different degrees, suggesting an intertwined regulation mechanism of wblA_so in morphological differentiation and secondary metabolism of S. somaliensis SCSIO ZH66.

Conclusions

wblA orthologues would be effective targets for activation of cryptic gene clusters in marine-derived Streptomyces strains, notwithstanding the regulation mechanisms might be varied in different strains. Moreover, the availability of the vio gene cluster has enriched the diversity of type III PKSs, providing new opportunities to expand the chemical space of polyketides through biosynthetic engineering.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0515-6) contains supplementary material, which is available to authorized users.

Specialised metabolites regulating antibiotic biosynthesis in Streptomyces spp

Streptomyces bacteria are the major source of antibiotics and other secondary metabolites. Various environmental and physiological conditions affect the onset and level of production of each antibiotic by influencing concentrations of the ligands for conserved global regulatory proteins. In addition, as reviewed here, well-known autoregulators such as γ-butyrolactones, themselves products of secondary metabolism, accumulate late in growth to concentrations allowing their effective interaction with cognate binding proteins, in a necessary prelude to antibiotic biosynthesis. Most autoregulator binding proteins target the conserved global regulatory gene adpA, and/or regulatory genes for 'cluster-situated regulators' (CSRs) linked to antibiotic biosynthetic gene clusters. It now appears that some CSRs bind intermediates and end products of antibiotic biosynthesis, with regulatory effects interwoven with those of autoregulators. These ligands can exert cross-pathway effects within producers of more than one antibiotic, and when excreted into the extracellular environment may have population-wide effects on production, and mediate interactions with neighbouring microorganisms in natural communities, influencing speciation. Greater understanding of these autoregulatory and cross-regulatory activities may aid the discovery of new signalling molecules and their use in activating cryptic antibiotic biosynthetic pathways.

Bafilomycins and Odoriferous Sesquiterpenoids from Streptomyces albolongus Isolated from Elephas maximus Feces

From a fermentation broth of Streptomyces albolongus obtained from Elephas maximus feces, nine bafilomycins (1-9) and seven odoriferous sesquiterpenoids (10-16) were isolated. The structures of the new compounds, including three bafilomycins, 19-methoxybafilomycin C1 amide (1), 21-deoxybafilomycin A1 (2), and 21-deoxybafilomycin A2 (3), and two sesquiterpenoid degradation products, (1β,4β,4aβ,8aα)-4,8a-dimethyloctahydronaphthalene-1,4a(2H)-diol (10) and (1β,4β,4aβ,7α,8aα)-4,8a-dimethyloctahydronaphthalene-1,4a,7(2H)-triol (11), were elucidated by comprehensive spectroscopic data analysis. The cytotoxicity activity against four human cancer cell lines and antimicrobial activities against a panel of bacteria and fungi of all compounds isolated were evaluated. Compounds 1, 7, and 8 were cytotoxic, with IC50 values ranging from 0.54 to 5.02 μM. Compounds 2, 7, 8, and 10 showed strong antifungal activity against Candida parapsilosis, with MIC values of 3.13, 1.56, 1.56, and 3.13 μg/mL respectively.

Marine natural products

This review covers the literature published in 2014 for marine natural products (MNPs), with 1116 citations (753 for the period January to December 2014) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1378 in 456 papers for 2014), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Rare actinomycetes Nocardia caishijiensis and Pseudonocardia carboxydivorans as endophytes, their bioactivity and metabolites evaluation

Two strains identified as Nocardia caishijiensis (SORS 64b) and Pseudonocardia carboxydivorans (AGLS 2) were isolated as endophytes from Sonchus oleraceus and Ageratum conyzoides respectively. The analysis of their extracts revealed them to be strongly bioactive. The N. caishijiensis extract gave an LC50 of 570 μg/ml(-1) in the brine shrimp cytotoxicity assay and an EC50 of 0.552 μg/ml(-1) in the DPPH antioxidant assay. Antimicrobial activity was observed against Methicillin resistant Staphlococcus aureus (MRSA) and Escherichia coli ATCC 25922 (14 mm), Klebsiella pneumoniae ATCC 706003 (13 mm), S. aureus ATCC 25923 (11 mm) and Candida tropicalis (20 mm). For the extract of P. carboxydivorans the EC50 was 0.670 μg/ml(-1) and it was observed to be more bioactive against Bacillus subtilis DSM 10 ATCC 6051 (21 mm), C. tropicalis (20 mm), S. aureus ATCC 25923 (17 mm), MRSA (17 mm), E. coli K12 (W1130) (16 mm) and Chlorella vulgaris (10 mm). The genotoxicity testing revealed a 20 mm zone of inhibition against the polA mutant strain E. coli K-12 AB 3027 suggesting damage to the DNA and polA genes. The TLC and bioautography screening revealed a diversity of active bands of medium polar and nonpolar compounds. Metabolite analysis by HPLC-DAD via UV/vis spectral screening suggested the possibility of stenothricin and bagremycin A in the mycelium extract of N. caishijiensis respectively. In the broth and mycelium extract of P. carboxydivorans borrelidin was suggested along with α-pyrone. The HPLC-MS revealed bioactive long chained amide derivatives such as 7-Octadecenamide, 9, 12 octadecandienamide. This study reports the rare actinomycetes N. caishijiensis and P. carboxydivorans as endophytes and evaluates their bioactive metabolites.

Gold catalysis in total synthesis – recent achievements

The most recent achievements of gold catalysed transformations applied in total synthesis of natural products are reviewed and analysed.

New α-pyrone and phthalide from the Xylariaceae fungus

A new α-pyrone xylaripyrone A (1) and a new phthalide xylariphthalide A (2) were isolated from the Xylariaceae fungus (no. 63-19-7-3), along with four related known phthalides (3-6): 4-[(acetyloxy)methyl]-7-methoxy-6-methyl-1(3H)-isobenzofuranone (3), convolvulol (4), 7-methoxy-4,6-dimethyl-3H-isobenzofuran-1-one (5), and convolvulanic acid B (6). Their structures were determined on the basis of IR, MS, and NMR spectroscopic analyses.

Effect of Culture Conditions on Metabolite Production of Xylaria sp

Seeking a strategy for triggering the cryptic natural product biosynthesis to yield novel compounds in the plant-associated fungus Xylaria sp., the effect of culture conditions on metabolite production was investigated. A shift in the production of five known cytochalasin-type analogues 1-5 to six new α-pyrone derivatives, xylapyrones A-F (compounds 6-11), from a solid to a liquid medium was observed. These compounds were identified by analysis of 1D and 2D NMR and HRMS data. Compounds 1-3 showed moderate cytotoxicity against HepG2 and Caski cancer cell lines with IC50 values ranging from 25 to 63 μM and compounds 4-11 were found to be inactive, with IC50 values>100 μM.

Recent advances in the synthesis of 2-pyrones

The present review summarizes the recent progresses in the synthesis of 2-pyrones and the application to the synthesis of marine natural products. Especially, much attention was placed on the transition metal catalyzed synthetic methodologies in this review.

A simple base-mediated synthesis of diverse functionalized ring-fluorinated 4H-pyrans via double direct C–F substitutions

A simple base-mediated synthesis of diverse substituted ring-fluorinated 4H-pyrans (monofluorinated 4H-pyrans) from trifluoromethylated alkenes and 1,3-dicarbonyl compounds was developed.

Synthesis and determination of absolute configuration of α-pyrones isolated from Penicillium corylophilum

The first total synthesis of (S)-6-(2,9-dihydroxynonyl)-4-hydroxy-3-methyl-2H-pyran-2-one, 4-hydroxy-3-methyl-6-((2S,4R)-2,4,11-trihydroxyundecyl)-2H-pyran-2-one, and its unnatural 2R,4R-isomer starting from commercially available 1,8-octanediol is described. The synthesis led to the revision of the proposed structural assignment of the natural product as (R)-6-(2,9-dihydroxynonyl)-4-hydroxy-3-methyl-2H-pyran-2-one. The key steps include chiral auxiliary mediated asymmetric acetate aldol reaction, dianion addition, and base mediated cyclization to form an α-pyrone ring.

Violapyrones H and I, new cytotoxic compounds isolated from Streptomyces sp. associated with the marine starfish Acanthaster planci

Two new α-pyrone derivatives, violapyrones H (1) and I (2), along with known violapyrones B (3) and C (4) were isolated from the fermentation broth of a marine actinomycete Streptomyces sp. The strain was derived from a crown-of-thorns starfish, Acanthaster planci, collected from Chuuk, Federated States of Micronesia. The structures of violapyrones were elucidated by the analysis of 1D and 2D NMR and HR-ESIMS data. Violapyrones (1-4) exhibited cytotoxicity against 10 human cancer cell lines with GI50 values of 1.10-26.12 μg/mL when tested using sulforhodamine B (SRB) assay. This is the first report on the cytotoxicity of violapyrones against cancer cell lines and the absolute configuration of violapyrone C.

One-pot synthesis and evaluation of novel 3-aryl-6-ethoxycarbonyl-4-hydroxy-2H-pyran-2-one as a potent cytotoxic agent

A series of new open chain analogs of Phelligridin J were synthesized and these compounds were found to be highly potent cytotoxic agents.